The present invention relates to a dispersion compensation apparatus and a dispersion compensation system which allows a reduction in the crosstalk when waves are multiplexed and demultiplexed in an optical transmission system, particularly in a wavelength multiplexing system.
It is known that the same medium refracts light of different wavelength differently. That is, velocity of light (to be precise, phase velocity) propagating through a medium depends upon the wavelength of the light. This phenomenon is referred to as dispersion. Dispersion brings a problem when an optical signal is transmitted using an optical fiber in particular or the like.
When transmission is performed in a state where light is confined within a part of a transmission medium like an optical fiber, a propagation velocity of a signal is also different depending on how the light is confined in the transmission medium, what is called a propagation mode (TE, TM, HE, EH, etc.). Propagation velocity differences (hereafter called Mode distribution) due to differences of the propagation modes occur in a multi-mode optical fiber which can propagate signals in a plurality of propagation modes at one wavelength.
On the other hand, a single-mode optical fiber which has only a basic mode as a propagation mode is mainly used for high-speed optical transmission because the mode distribution does not occur. However, in this single-mode optical fiber, the problem on the propagation velocity differences (hereafter called Material dispersion), caused by the fact that the refractive index depends on its wavelength, can not be avoided, so that propagating pulse widths lengthen (pulse distortion) caused by this material dispersion. That is, the dispersion becomes a cause of degradation in a signal waveform due to its transmission even in the single-mode optical fiber.
For example, when a 10 Gb/s-signal is to be transmitted, an allowable dispersion value is about 1000 ps/nm, which corresponds to the amount of dispersion in the single-mode optical fiber of about 70 km. Therefore, dispersion compensation becomes extremely important to perform long-haul transmission. As a device for dispersion compensation, a dispersion compensation fiber, for example, is commercially available. The dispersion compensation fiber is a special optical fiber that compensates for dispersion of a transmission line by inserting any optical fiber, that has a dispersion characteristic of sign opposite to a dispersion value indicating how an optical signal propagating inside the optical fiber is affected thereby, into the transmission line.
In the optical transmission using an optical fiber, it is generally highly efficient and preferable to perform wavelength multiplexing (WDM) transmission in which a transmission band is widened by the number of wavelengths to be used by simultaneously using a line of optical fiber at various wavelengths. The dispersion is a function of a wavelength, and its dispersion value is different in each wavelength. Therefore, in the WDM transmission, when a wavelength-multiplexed optical signal is to be transmitted by a single-mode optical fiber, each wavelength undergoes different dispersion. Therefore, as dispersion compensation for the wavelength-multiplexed optical signal, it is required to discretely compensate for a different dispersion value of each multiplexed wavelength.
With regard to dispersion compensation in such WDM transmission, there have been proposed, for example, xe2x80x9cWavelength multiplex transmissionxe2x80x9d disclosed in Japanese Patent Laid-Open Publication No. HEI09-116493 and xe2x80x9cWavelength dispersion compensation system for optical transmission linexe2x80x9d disclosed in Japanese Patent Laid-Open Publication No. HEI09-191290. These proposals are characterized in that each of the systems comprises a wavelength demultiplexing unit that demultiplexes a wavelength-multiplexed optical signal at each wavelength, and a dispersion compensation unit that independently compensates for wavelength dispersion of an optical signal due to an optical transmission line in each optical signal of demultiplexed wavelengths.
FIG. 11 is a block diagram showing an example of the conventional wavelength dispersion compensation system as disclosed in Japanese Patent Laid-Open Publication No. 09-116493. This figure shows a dispersion compensation system that performs respective dispersion compensation particularly on the following stage of an optical transmitter and the previous stage to an optical receiver.
In the wavelength dispersion compensation system shown in FIG. 11, at first, its transmission side comprises n units of transmitters 101A1 to 101An which oscillate different wavelengths from one another, dispersion compensation sections 102A1 to 102An provided on respective following stages of the transmitters 101A1 to 101An and each of which performs dispersion compensation for a wavelength oscillated by each of the transmitters, and an optical branching/coupling device (coupler) 100A that combines the optical signals transmitted through respective blocks comprising these transmitters 101A1 to 101An and dispersion compensation sections 102A1 to 102An. The optical signal combined in this optical branching/coupling device (coupler) 100A is transmitted to a transmission line 109.
On the other hand, its reception side comprises an optical branching/coupling device 100B that demultiplexes (separates) the wavelength into the wavelengths corresponding to those in the optical branching/coupling device 100A, band-pass filters 105B1 to 105Bn through which predetermined wavelengths pass, dispersion compensation sections 102B1 to 102Bn which perform dispersion compensation for the wavelengths of the optical signals having passed through the band-pass filters 105B1 to 105Bn, and receivers 101B1 to 101Bn which receive the optical signals transmitted through respective blocks consisting of these band-pass filters 105B1 to 105Bn and dispersion compensation sections 102B1 to 102Bn.
In FIG. 11, for example, the transmitter 101A1 oscillates a wavelength xcex1, and the band-pass filter 105B1 selectively passes an optical signal of the wavelength xcex1 therethrough. Accordingly, the dispersion compensation sections 102A1 and 102B1 perform dispersion compensation only for the optical signal of the wavelength xcex1. The dispersion compensation sections 102A1 and 102B1 can use a dispersion compensation fiber having a dispersion value of sign opposite to the dispersion value for the wavelength xcex1 indicated by the transmission line 109. The above configuration allows compensation for all the wavelengths in the transmitter side and the receiver side of the WDM transmission system so as to become zero dispersion.
FIG. 12 is block diagram showing the invention disclosed in Japanese Patent Laid-Open Publication No. HEI09-191290 as explained above, which is the diagram for explaining particularly a wavelength dispersion compensation system for an optical transmission line in a WDM transmission relay system.
The wavelength dispersion compensation system shown in FIG. 12 comprises transmission lines 119A and 119B which propagate optical signals of wavelengths xcex1 to xcexn multiplexed at n wavelengths, an optical branching/coupling device 110A which demultiplexes (separates) the wavelengths, band-pass filters 115A1 to 115An which pass predetermined wavelengths for optical signals of the separated wavelengths therethrough, dispersion compensation sections 112A1 to 112An which perform dispersion compensation for the wavelengths of the optical signals having passed through the band-pass filters 115A1 to 115An, and an optical branching/coupling device (coupler) 110B which combines again the optical signals transmitted through respective blocks consisting of these band-pass filters 115A1 to 115An and dispersion compensation sections 112A1 to 112An.
In the wavelength dispersion compensation system shown in FIG. 12, only the wavelength xcex1 having selectively passed through the band-pass filter 115A1 is input into the dispersion compensation section 112A1, and this dispersion compensation section 112A1 compensates for dispersion with respect to the wavelength xcex1 affected by the transmission lines 119A and 119B.
As explained above, according to xe2x80x9cWavelength dispersion compensation system for optical transmission linexe2x80x9d disclosed in Japanese Patent Laid-Open Publication No. HEI09-191290, wavelength dispersion compensation is performed for a wavelength-multiplexed optical signal at each wavelength of the optical signal, which makes it possible to perform simultaneous and perfect dispersion compensation for the wavelengths of all the optical signals whose wavelength dispersion is different.
Further, as another conventional example, the optical branching/coupling device 110A and the band-pass filters 115A1 to 115An forming a block and the optical branching/coupling device 110B shown in FIG. 12 can also be replaced with arrayed-waveguide gratings (AWG), respectively. FIG. 13 is a block diagram showing the conventional wavelength dispersion compensation system when these AWGs are used.
The wavelength dispersion compensation system shown in FIG. 13 comprises transmission lines 129A and 129B, two AWGs 120A and 120B, and dispersion compensation sections 124A1 to 124An for n wavelengths. The AWG 120A in particular separates an optical signal of a multiplexed wavelength input from an input port 121A1 into n wavelengths, and outputs the separated optical signals to output ports 122A1 to 122An, respectively.
The AWG 120B inputs the optical signals having passed through the dispersion compensation sections 124A1 to 124An from input ports 122B1 to 122Bn and also combines the signals to be output from an output port 121B1. These AWGs 120A and 120B have periodicity in their operations, and by utilizing this periodicity, optical signals can be combined or separated with their comparatively minimized size and low loss.
FIG. 14 is an explanatory diagram for explaining the periodicity of the AWG, and particularly shows the AWG which comprises input ports and output ports by N number respectively, and can combine or separate N wavelengths. In FIG. 14, when optical signals of wavelengths xcex1 to xcexN are successively input into input ports 1 to N of the AWG, the multiplex light of these xcex1 to xcexN can be obtained at an output port 1. Conversely, the multiplex light of xcex1 to xcexN is input into the output port 1, the optical signals of the wavelengths xcex1 to xcexN can be obtained in order from the input ports 1 to N.
When the optical signals of wavelengths xcexN, xcex1 to xcexNxe2x88x921 are successively input into the input ports 1 to N of the AWG, the multiplex light of these xcex1 to xcexN can be obtained at an output port 2. Conversely, the multiplexed light of xcex1 to xcexN is input into the output port 2, the optical signals of the wavelengths xcexN, xcex1 to xcexNxe2x88x921 can be obtained in order from the input ports 1 to N.
As explained above, the AWG generally has reversibility to input/output of optical signals to/from a plurality of input/output ports, and also has a certain relation, like the periodicity, between wavelengths of the optical signals input into the input ports and the output port which combines the optical signals of these wavelengths and outputs the combined signal. From these characteristics, the AWG makes wave combination and separation possible, and an input/output relations between respective input ports and output ports differs depending on which of the functions, wave combination and separation, the AWG is allowed to perform. In the explanation below, not depending on these functions, but it is assumed that a side to which an optical signal is input is an input port, while a side from which a combined or separated optical signal is output is an output port.
Crosstalk of such an AWG is explained below. The AWG is commonly used as a wave combiner/divider in a current wavelength multiplexing system, but crosstalk between adjacent wavelengths can not be neglected in association with higher density in multiplexing of wavelengths. FIG. 15 is an explanatory diagram for explaining the crosstalk in the AWG.
In FIG. 15, an AWG 140A has N number of input ports 141A1 to 141AN and N number of output ports 142A1 to 142AN having the periodicity as shown in FIG. 14. For example, when multiplex light of wavelengths xcex1, xcex2, and xcex3 is input into an input port 141A3, the light is separated into the wavelengths xcex3, xcex2, and xcex1, which are output in order from the output ports 142A1 to 142A3 based on the periodicity shown in FIG. 14.
As explained above, in the wavelength dispersion compensation system, by applying the AWG to a portion that performs a function of combining and separating optical signals, the dispersion compensation apparatus that performs dispersion compensation, that is, the configuration comprising the dispersion compensation sections 124A1 to 124An, the wave combiner (AWG 120A), and the wave divider (AWG120B) shown in FIG. 13 can be simplified.
As another related conventional example, xe2x80x9cLIGHT AMPLIFYING APPARATUSxe2x80x9d disclosed in U.S. Pat. No. 5,510,930 stabilizes the operation of an optical amplifier by polarizing pump light to a right-circularly polarized wave and left-circularly polarized wave. xe2x80x9cDISPERSION COMPENSATION DEVICExe2x80x9d disclosed in U.S. patent application Ser. No. 207,419 performs efficient dispersion compensation for wavelength-multiplexed light by using a diffraction grating whose both edges have different grating pitches.
In the conventional dispersion compensation apparatus, however, since dispersion compensation is discretely performed for each wavelength, dispersion compensation sections are required by the number of wavelengths to be combined or separated, accordingly, there has been a problem such that the scale of a circuit increases, which makes the circuit complicated and costly.
There has been also a problem such that wavelengths output from adjacent output ports are included as crosstalk in an optical signal output from each of the output ports of the AWGs or the like. For example, as shown in FIG. 15, as an optical signal output from the output port 142A2, only an optical signal of a wavelength xcex2 should originally be output, but wavelengths xcex1 and xcex3 output from the output ports 142A1 and 142A3 adjacent to the output port 142A2 are partially included in the optical signal output from the output port 142A2. Accordingly, there have been problems such that signal quality is degraded, which exerts a bad effect on transmission characteristics and reception characteristics of the optical transmission system.
It is therefore an object of this invention, in an optical transmission system, to provide a dispersion compensation apparatus which has high-reliability transmission characteristics and reception characteristics and can also achieve minimization of the apparatus and cost reduction, and also to provide a dispersion compensation system.
A dispersion compensation apparatus according to this invention is characterized in that, this dispersion compensation apparatus that performs dispersion compensation in an optical transmission system, the apparatus comprises a first wave combiner which receives a plurality of optical signals having different wavelengths, divides the received optical signals into a plurality of groups, and combines the optical signals included in each of the divided groups to output a first multiplex light corresponding to each of the groups; and a second wave combiner which receives the plurality of first multiplex lights output from the first wave combiner, and combines the first multiplex lights to output a second multiplex light.
According to the invention, when a plurality of optical signals having different wavelengths from each other are input to obtain multiple light of the signals (second multiplex light), at first, the first wave combiner divides the plurality of optical signals into a plurality of groups, and combines the optical signals in each of the divided groups to output the first multiplex light in each of the groups, and then the second wave combiner outputs final multiplex light, therefore, it is possible to perform dispersion compensation and band-passing on the first multiplex lights smaller in number than the number of initially input optical signals, thus it is possible to perform high-reliability optical transmission with a simpler configuration.
A dispersion compensation apparatus according to this invention is characterized in that, in the above explained dispersion compensation apparatus, the first wave combiner has a leakage suppression unit which receives the optical signals in each of the groups and suppresses leakage of optical signals included in adjacent groups.
According to the invention, the first wave combiner has the leakage suppression unit which receives the plurality of optical signals in each of the groups and suppresses leakage (crosstalk or the like) of the optical signals included in the adjacent groups, therefore, it is possible to obtain first multiplex light from which any unnecessary wavelength components have been removed.
A dispersion compensation apparatus according to this invention is characterized in that, in the above dispersion compensation apparatus, the leakage suppression unit has at least one leakage signal output terminal which is provided between output sections for outputting multiplex light in each of the groups, and outputs leakage of the optical signals included in the adjacent groups.
According to the invention, the first wave combiner has the leakage output section which becomes an outlet of the leakage signal as the leakage suppression unit which receives a plurality of optical signals in each group and suppressing leakage (crosstalk or the like) of the optical signals included in the adjacent groups, therefore, it is possible to process any unnecessary wavelength components input from the adjacent groups as ineffective ones.
A dispersion compensation apparatus according to this invention is characterized in that, the above explained dispersion compensation apparatus comprises a dispersion compensation unit which subjects each of the first multiplex lights output from the first wave combiner to compensation for dispersion of predetermined wavelengths.
According to the invention, there are provided the dispersion compensation units which subject the respective first multiplex lights output from the first wave combiner to compensation for dispersion of the predetermined wavelengths, therefore, there is no need to discretely provide the dispersion compensation units such as dispersion compensation fibers for the respective optical signals input into the first wave combiner.
A dispersion compensation apparatus according to this invention is characterized in that, the above explained dispersion compensation apparatus comprises a filter unit provided on the previous stage to the dispersion compensation unit, wherein the filter unit allows only an optical signal of a predetermined wavelength to passes through.
According to the invention, the filter unit such as a band-pass filter, which passes only an optical signal of a predetermined wavelength therethrough, is provided on the previous stage to the dispersion compensation unit, therefore, any unnecessary wavelength components are removed more effectively, thus it is possible to obtain only multiplex light (first multiplex light) within a range of target wavelengths.
A dispersion compensation apparatus according to this invention is characterized in that, in the above explained dispersion compensation apparatus, the first wave combiner is an arrayed-waveguide grating.
According to the invention, the first wave combiner is the arrayed-waveguide grating, therefore, the unit can be produced compactly as a part of a planar lightwave circuit.
A dispersion compensation apparatus according to this invention is characterized in that, this dispersion compensation apparatus that performs dispersion compensation in an optical transmission system, the apparatus comprises a first wave divider which receives first multiplex light including a plurality of optical signals having different wavelengths, and separates the input first multiplex light to output a plurality of second multiplex lights; and a second wave divider which receives the second multiplex lights output from the first wave divider, and separates each of the input second multiplex lights to output a plurality of optical signals included in each of the second multiplex lights.
According to the invention, when a plurality of optical signals having different wavelengths from each other included in the multiplex light (first multiplex light) are to be obtained, at first, the first wave divider separates the first multiplex light to output a plurality of second multiplex lights, and the second wave divider outputs a plurality of final optical signals included in each of the second multiplex lights, therefore, it is possible to perform dispersion compensation and band-passing on the second multiplex lights smaller in number than the number of optical signals to be finally output, thus it is possible to perform high-reliability optical transmission with a simpler configuration.
A dispersion compensation apparatus according to this invention is characterized in that, in the above explained dispersion compensation apparatus, the second wave divider has a leakage suppression unit which suppresses leakage of adjacent multiplex lights at the time of inputting the second multiplex lights.
According to the invention, the second wave divider has the leakage suppression unit which receives the first multiplex light and suppressing leakage (crosstalk or the like) of adjacent multiplex lights, therefore, it is possible to obtain second multiplex light from which any unnecessary wavelength components have been removed.
A dispersion compensation apparatus according to this invention is characterized in that, in the above explained dispersion compensation apparatus, the leakage suppression unit has at least one leakage signal output terminal which is provided between output sections for outputting multiplex light in each of the groups, and outputs leakage of the optical signals included in the adjacent groups.
According to the invention, the second wave divider has the leakage output section, which becomes an outlet of the leakage signal, as the leakage suppression unit which receives a plurality of optical signals in each group and suppressing leakage (crosstalk or the like) of the optical signals included in the adjacent groups, therefore, it is possible to process any unnecessary wavelength components input from the adjacent groups as ineffective ones.
A dispersion compensation apparatus according to this invention is characterized in that, the above explained dispersion compensation apparatus comprises a dispersion compensation unit which subjects each of the second multiplex lights output from the first wave divider to compensation for dispersion of predetermined wavelengths.
According to the invention, there are provided the dispersion compensation units which subject the respective second multiplex lights output from the first wave divider to compensation for dispersion of the predetermined wavelengths, therefore, there is no need to discretely provide the dispersion compensation units such as dispersion compensation fibers for the respective optical signals to be finally output from the second wave divider.
A dispersion compensation apparatus according to this invention is characterized in that, the above explained dispersion compensation apparatus comprises a filter unit provided on the previous stage to the dispersion compensation unit, wherein the filter unit allows only an optical signal of a predetermined wavelength to passes through.
According to the invention, the filter unit such as a band-pass filter, which passes only an optical signal of a predetermined wavelength therethrough, is provided on the previous stage to the dispersion compensation unit, therefore, any unnecessary wavelength components are removed more effectively, thus it is possible to obtain only multiplex light (second multiplex light) within a range of target wavelengths.
A dispersion compensation apparatus according to this invention is characterized in that, in the above explained dispersion compensation apparatus, the second wave divider is an arrayed-waveguide grating.
According to the invention, the second wave divider is the arrayed-waveguide grating, therefore, the unit can be produced compactly as a part of a planar lightwave circuit.
A dispersion compensation apparatus according to this invention is characterized in that, this dispersion compensation apparatus that performs dispersion compensation in an optical transmission system, the apparatus comprises a wave combiner/divider which receives a plurality of optical signals having different wavelengths, divides the input optical signals into a plurality of first groups, combines the optical signals included in each of the first groups to be output as first multiplex light, receives a plurality of second multiplex lights, and separates each of the input second multiplex lights to output a plurality of optical signals included in the second multiplex light in each second group, the combiner/divider having input terminals which receives the optical signals of the first groups and output terminals for outputting the optical signals of the second groups, wherein the input terminals and output terminals are arranged alternately and adjacent to each other; and the combiner/divider further having output terminals for outputting the first multiplex light and input terminals which receives the second multiplex light, wherein the input terminals and output terminals are arranged alternately and adjacent to each other.
According to the invention, there is provided the wave combiner/divider which receives a plurality of optical signals having different wavelengths from each other, divides the plurality of input optical signals into a plurality of first groups, combines the optical signals included in each of the first groups to be output as first multiplex light, receives a plurality of second multiplex lights, and separates each of the input second multiplex lights to output a plurality of optical signals included in the second multiplex light in each second group; in which the input sections which receives the optical signals of the first groups and the output sections for outputting the optical signals of the second groups are arranged alternately and adjacent to each other, and also the output sections for outputting the first multiplex light and the input sections which receives the second multiplex lights are arranged alternately and adjacent to each other, therefore, it is possible to concurrently perform transmission of multiplex light formed with a plurality of optical signals and reception of a plurality of optical signals from the multiplex light, and also to perform dispersion compensation and band-passing on the first or the second multiplex lights smaller in number than the number of initially input optical signals or the number of optical signals to be finally output, thus it is possible to perform high-reliability optical transmission with a simpler configuration.
A dispersion compensation apparatus according to this invention is characterized in that, the above explained dispersion compensation apparatus comprises a dispersion compensation unit which subjects each of the first and second multiplex lights to compensation for dispersion of predetermined wavelengths.
According to the invention, there are provided the dispersion compensation units which subject the respective first multiplex lights and second multiplex lights to compensation for dispersion of the predetermined wavelengths, therefore, there is no need to discretely provide the dispersion compensation units such as dispersion compensation fibers for the respective optical signals to be initially input or optical signals to be finally output to or from the wave combiner/divider.
A dispersion compensation apparatus according to this invention is characterized in that, the above explained dispersion compensation apparatus has optical isolators provided on the following stage of the output terminal for outputting the first multiplex light and on the previous stage to the input terminal which receives the second multiplex light, respectively.
According to the invention, the optical isolators are provided on the following stage of the output section for outputting the first multiplex light and the previous stage to the input section which receives the second multiplex light, respectively, therefore, it is possible to prevent optical signals in a reverse direction from being mixed into these output sections and input sections, thus it is possible to ensure the direction of optical signals between the adjacent input/output sections.
A dispersion compensation system according to this invention is characterized in that, in this dispersion compensation system for performing respective dispersion compensation in a transmission unit and a reception unit in an optical transmission system, the above explained dispersion compensation apparatus is provided in the transmission section, and the above explained dispersion compensation apparatus is provided in the reception section.
According to the invention, the dispersion compensation system is constructed by providing the specific dispersion compensation apparatuses in the transmission section and the reception section, therefore, it is possible to enjoy the advantages of the dispersion compensation apparatus which can perform high-reliability optical transmission with a simpler configuration.
A dispersion compensation system according to this invention is characterized in that, in this dispersion compensation system for performing respective dispersion compensation in a transmission unit and a reception unit in an optical transmission system, the above explained dispersion compensation apparatuses are provided in the transmission section and in the reception section, respectively.
According to the invention, the dispersion compensation system is constructed by providing the specific dispersion compensation apparatuses in the transmission section and the reception section respectively, therefore, it is possible to enjoy the advantages of the dispersion compensation apparatus which can perform high-reliability optical transmission with a simpler configuration.